A friction transmission belt such as a V-belt, a V-ribbed belt or a flat belt, and a synchronous power transmission belt such as a toothed belt are conventionally known as a transmission belt transmitting power. Those transmission belts have a core wire embedded in a rubber body along a lengthwise direction of the belt and this core wire plays a role of transmitting power from a drive pulley to a driven pulley. Such transmission belts are generally provided with an adhesive rubber layer in order to enhance adhesiveness between the core wire and a rubber.
Patent Document 1 discloses a rubber V-belt containing an extensible rubber layer and a compressed rubber layer each having short fibers having high elastic modulus arranged in a width direction of the belt, provided on the upper and lower sides of an adhesive rubber layer having a cord embedded therein, in which the adhesive rubber layer is constituted of a rubber composition containing 100 parts by weight of a chloroprene rubber, from 1 to 20 parts by weight of at least one metal oxide vulcanizing agent selected from zinc oxide, magnesium oxide and lead oxide, from 5 to 30 parts by weight of silica, from 15 to 50 parts by weight of a reinforcing filler, and from 2 to 10 parts by weight of bismaleimide. It is described that in this rubber V-belt, crosslinking density can be increased by compounding bismaleimide to thereby form an adhesive rubber having high elastic modulus, therefore stress concentration between the adhesive rubber and a fiber-containing rubber (a compressed rubber or an extensible rubber) is decreased, and additionally, since the adhesive rubber layer has excellent fatigue resistance, belt life can be prolonged.
However, in a layout in which a belt greatly bends and a load is high (for example, a state that a belt moves inward in a radius direction of a pulley and the belt greatly bends, like in a variable speed belt, or a state that a belt is attached by bending to a plurality of pulleys, like in a V-ribbed belt), a mere increase in elastic modulus (rubber hardness) of an adhesive rubber layer is not sufficient to prevent interfacial peeling between the adhesive rubber layer and a compressed rubber layer (or an extensible rubber layer) and peeling between a core wire and the adhesive rubber layer. Furthermore, in the case where rubber hardness of the adhesive rubber layer is excessively increased, there is a possibility that bending fatigue resistance is deteriorated.
On the other hand, in the case where rubber hardness of an adhesive rubber layer is merely decreased (for example, crosslinking density is decreased by decreasing the amount of a reinforcing filler added or using a smaller amount of a vulcanization type compounding ingredient) for the purpose of the improvement in bending fatigue resistance and adhesiveness, great difference is generated in rubber hardness between a compressed rubber layer (or an extensible rubber layer) and the adhesive rubber layer, and peeling occurs early at the interface between the adhesive rubber layer and the compressed rubber layer (or the extensible rubber layer). For this reason, it was difficult in the conventional technique to prevent interfacial peeling and improve durability without deterioration of bending fatigue resistance.